In an injection molding machine, a plastic material plasticized in an elongated cylinder is injected through a nozzle by rotational movement of a screw to fill the cavity of a mold with the plastic material. Thereafter, when the plastic material in the mold is cooled and set, the molding is taken out by opening the mold.
A programmed injection process which controls the injection velocity is known as a conventional technique according to an injection control method in an injection molding machine of this type. This method includes instead controlling the moving velocity of the screw in order to control the injection velocity from the nozzle, namely, the flow velocity of the resin into the mold wherein the moving velocity of the screw is controlled in a plurality of stages. This technique is disclosed in, for example, Published Unexamined Japanese Patent Applications (Tokkaisho) Nos. 48-95458 and 59-64337.
In these conventional techniques, however, since the diameter of the nozzle opening is constant, a change in the injection velocity causes the heating value produced by shearing of the resin at the nozzle to be changed. Thus even if only the injection velocity is desired to be changed, the resin temperature is also changed together, so that the controlled factors are complicated and control is difficult.
Another conventional technique (Published Unexamined Japanese Patent Application (Tokkaisho) No. 60-242022 discloses a throttled path portion provided at the nozzle end. The opening of the throttled path portion is changed in injection to change the heating value produced by shearing at the nozzle to thereby provide a desired resin temperature. In order to only eliminate uneven distribution of temperature of the resin varying depending on the position of the screw stroke, this technique variably controls the opening of the nozzle valve in accordance with the position of the screw to thereby maintain the temperature of the injected resin at a constant temperature. However, in order to provide such control, it is necessary to accurately understand at all times the distribution of the resin temperature along the axis of the screw before the injection step starts. To this end, a troublesome pre-process must be performed which includes sensing the distribution of the resin temperature, knowing the average distribution of temperature by many experiments, etc., so that this technique is problematic from a standpoint of accurate control of temperature, working efficiency and practicality.
For control of even resin temperature in injection molding, the factors which change the resin temperature include the temperature of a mold used in addition to the heating due to shearing at the nozzle section which is due to a change in the injection velocity. If the temperature of the mold fluctuates, the temperature of the melted resin filled in the cavity fluctuates correspondingly, the specific volume of the resin also fluctuates, and hence the resulting product fluctuates in dimension.
As a prior art which has percieved the temperature of a mold, there is known a molding control method which includes the steps of calculating the correlation between the temperature (or the resin temperature) and the resin pressure, sensing the actual mold temperature (or the actual resin temperature) in the injection molding and controlling the resin pressure. For example, the pressure is decreased in order to decrease the resin pressure by a quantity corresponding to a decrease in the mold temperature (or the resin temperature).
Such molding control method is much troublesome, its accuracy is low and not an optimal molding control method. Namely, if the resin pressure is changed, the distribution of internal stress in the mold will change and bad products may result undesirably.
If the mold temperature (or resin temperature) fluctuates, it is conceivable to control the temperature of the housing in accordance with that fluctuation to directly correct the resin temperature, but the temperature responsive rate of the housing is greatly low compared to molding cycle rate, so that such control is not effective practically.
If the mold temperature mold is controlled precisely so as not to fluctuate, stabilized molding can be performed. However, the mold temperature varies depending on the atmospheric temperature, so that the temperature within a factory in which the injection molding machine is installed must be controlled at a constant temperature in order to control the mold temperature precisely, and therefore the control would be expensive and not practical.
The present invention derives from the contemplation of such situations. It is an object of the present invention to provide a control apparatus for an injection molding machine which is capable of controlling the injection velocity and the resin temperature separately to thereby facilitate the molding operation, and to manufacture precise and uniform moldings.
It is another object of the present invention to provide a control apparatus for an injection molding machine which is capable of maintaining the specific volumes of resin uniformly at all times at injection irrespective of fluctuations of the mold temperature to thereby provide precise stabilized moldings successively.